This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in French Patent Application No. 01 05484 filed on Apr. 24, 2001.
The invention relates in particular to apparatus for use in vehicle body building to measure deformations or to verify that bodywork assemblies are properly positioned relative to the remainder of a structure.
In this field, measurement devices are known of the type comprising a base having a hinged arm connected thereto, the arm having an end support carrying a feeler ball.
Each hinge of the arm is fitted with an encoder, so that it suffices to collect the measurements from each of the encoders in order to determine the orientation of the end support and the position of the feeler ball in a frame of reference associated with the base.
That type of apparatus is transportable, but it is sometimes heavy and bulky, and even when folded the arm projects considerably from the base.
Furthermore, when deployed, the arm is fragile and can be twisted during clumsy handling, thereby completely falsifying any determination of the orientation of the end support or of the position of the feeler ball, thus making the apparatus unusable.
Finally, the presence of numerous mechanical parts and high precision hinges makes that type of apparatus expensive to produce.
Document EP-A-1 003 013 describes a method of determining the position of a reference mark associated with a camera by taking a image of the reflections in a mirror of reference marks associated with the camera. That method is used for identifying the position and the orientation of the wheels of a motor vehicle.
In analogous manner, document DE-A-4 041 723 describes a method of identifying the position of a reference mark by means of a plurality of cameras placed to form pairs of cameras looking at each other, with one of the cameras in each pair of cameras being secured to a camera in an adjacent pair. The major drawback of those methods lies in the lack of accuracy with which distances are measured by analyzing images, which puts a de facto limit on the accuracy with which precision can be identified.
To complete the technological background of the invention, mention is made of document U.S. Pat. No. 5,148,591 which teaches a method of optically guiding a handling robot, of documents U.S. Pat. No. 4,513,504 and EP-A-0 149 690 which teach determining the curvature of a pipe by means of a plurality of cameras placed one after another, and finally of document WO-A-98/48241 (U.S. Pat. No. 6,279,246) which teaches determining the position of a point with the help of an object which is moved into various positions.
An object of the invention is to provide a measurement method and apparatus for implementing the method that do not present the above-specified drawbacks.
To this end, the invention provides a measurement method for determining the position and the orientation of a moving assembly in a given frame of reference, the method comprising the following steps:
taking an image of an optical reference mark associated with the moving assembly by means of a first camera having a focal axis lying in a direction that is known in the base frame of reference;
taking an image of an optical reference mark associated with the first camera by means of a second camera associated with the moving assembly;
measuring the distance between a point associated with the first camera and a point associated with the second camera by using self-contained measurement means that are independent of said first and second cameras; and
computing the position and the orientation of the moving assembly in the base frame of reference on the basis of the images taken and of the measured distance.
The images of the optical reference marks make it possible to determine relative rotation between the two optical reference marks and also relative offset between the two optical reference marks in a plane normal to the focal axis of the first camera. Distance measurement makes it possible to determine the distance between the two optical reference marks, and to do so very accurately because of the self-contained nature of the measurement means used which does not depend on the cameras, unlike the previously known methods outlined above.
The relative position and orientation of the second optical reference mark are thus known relative to the first optical reference mark.
The position of the first optical reference mark is associated with the direction of the focal axis of the first camera. The position and the orientation of the moving assembly can thus be deduced in the given base frame of reference.
For a moving assembly fitted with a feeler ball, the above method is advantageously enriched by an additional step which consists in determining the position of the center of the feeler ball in the base frame of reference.
The invention also provides measurement apparatus for implementing the above method, the apparatus comprising a first assembly carrying the first camera and the associated optical reference marks, and a second assembly carrying the second camera and the associated optical reference marks, said apparatus further including measurement means for measuring the distance between a point of the first assembly and a point of the second assembly, said measurement means being self-contained and independent of said first and second cameras, and a computer interfaced with both cameras and with the distance-measuring means, said computer being arranged to compute the position and the orientation of the moving assembly in the base frame of reference.
Preferably, the second moving assembly is also fitted with a feeler ball, and the computer is arranged to determine the position of the center of the feeler ball in the base frame of reference.
In which case, advantageously, the second assembly includes a handle-forming support having the second camera and the associated optical reference mark fitted thereto, together with the feeler ball.
Also advantageously, the second assembly can be detachably received by the first assembly. This makes the apparatus easier to transport. Similarly, the computer is advantageously carried by the first assembly.
Preferably, the first assembly comprises a stand standing on the ground together with a cradle movably mounted on the stand and carrying the first camera and the associated optical reference mark.
It is then advantageous for the cradle to be associated with the stand by a cardan type mount.
Also preferably, the self-contained measurement means comprises a winder fitted to the first assembly, the winder receiving a line having one end attached to a point of the second assembly. The winder is then advantageously fitted to the cradle.
Finally, the apparatus is preferably fitted with signaling means for indicating whether the optical reference mark associated with the second camera is in the field of view of the first camera while the optical reference mark associated with the first camera is simultaneously in the field of view of the second camera.